Methods of manufacturing electronic devices involving depositing active components from solution are known in the art. Such methods involve the preparation of a substrate onto which one or more active components can be deposited. If active components are deposited from solution, one problem is how to contain the active components in desired areas of the substrate. One solution to this problem is to provide a substrate comprising a patterned bank layer defining wells in which the active components can be deposited in solution. The wells contain the solution while it is drying such that the active components remain in the areas of the substrate defined by the wells.
The aforementioned solution processing methods have been found to be particularly useful for deposition of organic materials in solution. The organic materials may be conductive, semi-conductive, and/or opto-electrically active such that they can emit light when an electric current is passed through them or detect light by generating a current when light impinges on them. Devices which utilize these materials are known as organic electronic devices. An example is an organic transistor device. If the organic material is a light-emissive material the device is known as an organic light-emissive device. Transistors and light emissive devices are discussed in more detail below.
Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semi-conductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semi-conductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semi-conductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semi-conductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and injecting holes or electrodes.
For example, a p-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO level of the semi-conductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO level of the semi-conductive material can enhance electron injection and acceptance. Ambipolar devices that can function as n-type or p-type devices are also known.
Transistors can be formed by depositing the components in thin films to form a thin film transistor (TFT). When an organic material is used as the semi-conductive material in such a device, it is known as an organic thin film transistor (OTFT).
Various arrangements for organic thin film transistors are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semi-conductive material disposed therebetween in a channel region, a gate electrode disposed adjacent the semi-conductive material and a layer of insulting material disposed between the gate electrode and the semi-conductive material in the channel region.
OTFTs may be manufactured by low cost, low temperature methods such as solution processing. Moreover, OTFTs are compatible with flexible plastic substrates, offering the prospect of large-scale manufacture of OTFTs on flexible substrates in a roll-to-roll process.
An example of such an organic thin film transistor is shown in FIG. 1. The illustrated structure may be deposited on a substrate 1 and comprises source and drain electrodes 2, 4 which are spaced apart with a channel region 6 located therebetween. An organic semiconductor (OSC) 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4. An insulating layer 10 of dielectric material is deposited over the organic semi-conductor 8 and may extend over at least a portion of the source and drain electrodes 2, 4. Finally, a gate electrode 12 is deposited over the insulating layer 10. The gate electrode 12 is located over the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The structure described above is known as a top-gate organic thin film transistor as the gate is located on a top side of the device. Alternatively, it is also known to provide the gate on a bottom side of the device to form a so-called bottom-gate organic thin film transistor.
An example of such a bottom-gate organic thin film transistor is shown in FIG. 2. In order to more clearly show the relationship between the structures illustrated in FIGS. 1 and 2, like reference numerals have been used for corresponding parts. The bottom-gate structure illustrated in FIG. 2 comprises a gate electrode 12 deposited on a substrate 1 with an insulating layer 10 of dielectric material deposited thereover. Source and drain electrodes 2, 4 are deposited over the insulating layer 10 of dielectric material. The source and drain electrodes 2, 4 are spaced apart with a channel region 6 located therebetween over the gate electrode. An organic semiconductor (OSC) 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
One problem with the aforementioned arrangements is how to contain the OSC within the channel region when it is deposited. A solution to this problem is to provide a patterned layer of insulating bank material 14 defining a well in which the OSC 8 can be deposited from solution by, for example, inkjet printing. Such an arrangement is shown in FIGS. 3 and 4 for bottom and top gate organic thin film transistors respectively. Again, in order to more clearly show the relationship between the structures illustrated in FIGS. 1 and 2, with those illustrated in FIGS. 3 and 4, like reference numerals have been used for corresponding parts.
The periphery of the well defined by the patterned layer of insulating material 14 surrounds some or all of the channel 6 defined between the source and drain electrodes 2, 4 in order to facilitate deposition of the OSC 8 by, for example, inkjet printing. Furthermore, as the insulating layer 14 is deposited prior to deposition of the OSC 8, it may be deposited and patterned without damaging the OSC. The structure of the insulating layer 14 can be formed in a reproducible manner using known deposition and patterning techniques such as photolithography of positive or negative resists, wet etching, dry etching, etc.
Even if a patterned layer of well-defining bank material is provided, problems still exist in containing the OSC within the channel region and providing good film formation of the OSC in the channel region using solution processing techniques for deposition of the OSC. Uncontrollable wetting of the well-defining bank layer may occur since the contact angle of the OSC solution on the well-defining bank layer is typically low. In the worst case the OSC may overspill the wells.
One solution is to treat the surface of the well-defining bank using, for example, a fluorine based plasma such as CF4 in order to reduce its wettability prior to depositing the OSC from solution. A de-wetting surface on the top of the well-defining bank layer aids in containing the OSC within the wells when it is deposited.
Another solution is to use an inherently low-wetting material for the well-defining bank layer. US 2007/0023837 describes an arrangement in which a low-wetting fluorine containing polymer such as “Cytop” made by Asahi Glass in Japan is used to form a patterned well-defining bank layer when manufacturing a TFT substrate. The low-wetting fluorine containing polymer material is good at preventing the OSC from over-spilling the wells when deposited from solution. However, as the sides of the well are also low-wetting, the solution tends to be contained on the base of the well leading to poor film formation. That is, because the solution of OSC doesn't wet the sides of the well it forms a curved drop on the base of the well and dries to form a film of uneven thickness. Films of uneven thickness can adversely effect the performance of a resultant device as is known in the art.
US 2007/0020899 discloses treating the surface of a bank layer defining a wiring pattern for an electronic substrate using a fluorine based plasma in order to reduce its wettability as discussed previously. This document also describes an alternative method in which a two-layer bank structure is provided which defines a wiring pattern for an electronic substrate. The two-layer bank structure comprising a first layer which has good wettability and a second layer thereover comprising a low-wetting fluorine containing polymer.
With the aforementioned two-layer bank structure, a liquid deposited in the wells can wet the sides of the wells made of the first layer to provide good film formation in the wells on drying whereas the second layer prevents the liquid from over-spilling the wells. The document suggests that the materials for both the first and second bank layers should be polymers including siloxane bonds in a main chain and the polymer of the second bank layer should include fluorine bonds in a side chain. Materials for the second bank layer are described as having contact angles of 50° and above. A manufacturing process is also disclosed in which the two layer bank structure is formed, active component is deposited in wells defined by the bank structure, and then the active component and the bank structure are baked at the same time.
The aforementioned prior art document describes that conductive particles can be dispersed in a dispersion medium and deposited into areas defined by the two-layer bank structure by inkjet printing in order to form conductive circuitry. It is described that the conductive particles can be a metal, an oxide, an alloy, an organometallic compound or a conductive polymer. Various dispersion mediums are listed including water, alcohols, hydrocarbon compounds and ether compounds.
The document describes that the aforementioned method can be used to manufacture back plane circuitry for a display. According to the document, a TFT substrate is provided on which the two-layer bank structure is then deposited and used to define areas in which pixel electrodes are deposited by inkjet printing conductive particles of, for example, ITO in a dispersion medium. It is described that the substrate can be used as a back plane for a liquid crystal display or an organic electroluminescent device. For the organic electroluminescent device two separate bank structures appear to be disclosed: a first bank structure defining pixel electrodes which is formed using the previously described two-layer bank structure; and a second bank structure defining wells in which hole transporting material and light emissive material are inkjet printed, this second bank structure consisting of a single layer which is not treated in any way.
The aforementioned prior art relates to the provision of low-wettability banks for the manufacture of TFT substrates although the use of single bank layer structures for light emissive materials is also mentioned. Organic light emissive devices are discussed in more detail below.
Displays fabricated using OLEDs (organic light emitting devices) provide a number of advantages over other flat panel technologies. They are bright, colourful, fast-switching, provide a wide viewing angle, and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) light emitting diodes (LEDs) may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colours which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160. Examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343. Examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
OLEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixelated display. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a thin film transistor (TFT), associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Other passive displays include segmented displays in which a plurality of segments share a common electrode and a segment may be lit up by applying a voltage to its other electrode. A simple segmented display need not be scanned but in a display comprising a plurality of segmented regions the electrodes may be multiplexed (to reduce their number) and then scanned.
FIG. 5a shows a vertical cross section through an example of an OLED device 100. In an active matrix display, part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 5a). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer 104 is deposited on the substrate, typically comprising around 40 to 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500 nm of aluminum, or a layer of aluminum sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are widely available. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching.
A substantially transparent hole injection layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole injection layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate doped polyethylene-dioxythiophene) from H. C. Starck of Germany. In a typical polymer-based device the hole transport layer 106 may comprise around 200 nm of PEDOT. The light emitting polymer layer 108 is typically around 70 nm in thickness. These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing. In this latter case, banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited. Such wells define light emitting areas or pixels of the display.
Cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapor deposition) covered with a thicker, capping layer of aluminum. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may be achieved or enhanced through the use of cathode separators (not shown in FIG. 5a).
The same basic structure may also be employed for small molecule devices.
Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress. Alternatively, the displays can be encapsulated prior to scribing and separating.
To illuminate the OLED, power is applied between the anode and cathode by, for example, battery 118 illustrated in FIG. 5a. In the example shown in FIG. 5a light is emitted through transparent anode 104 and substrate 102 and the cathode is generally reflective. Such devices are referred to as “bottom emitters”. Devices which emit through the cathode (“top emitters”) may also be constructed, for example, by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent and/or using a transparent cathode material such as ITO.
Referring now to FIG. 5b, this shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of FIG. 5a are indicated by like reference numerals. As shown, the hole transport layer 106 and the electroluminescent layer 108 are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode metal 104 and cathode layer 110 respectively. In the figure conductive lines 154 defined in the cathode layer 110 run into the page and a cross-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown. An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode metal layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs).
The above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture. The device is therefore encapsulated in a metal or glass can 111, attached by UV-curable epoxy glue 113 onto anode metal layer 104. Preferably the anode metal contacts are thinned where they pass under the lip of the metal can 111 to facilitate exposure of glue 113 to UV light for curing.
Considerable effort has been dedicated to the realization of a full-color, all plastic screen. The major challenges to achieving this goal have been: (1) access to conjugated polymers emitting light of the three basic colors red, green and blue; and (2) the conjugated polymers must be easy to process and fabricate into full-color display structures. Polymer light emitting devices (PLEDs) show great promise in meeting the first requirement, since manipulation of the emission color can be achieved by changing the chemical structure of the conjugated polymers. However, while modulation of the chemical nature of conjugated polymers is often easy and inexpensive on the lab scale it can be an expensive and complicated process on the industrial scale. The second requirement of easy processability and build-up of full-color matrix devices raises the question of how to micro-pattern fine multicolor pixels and how to achieve full-color emission. Inkjet printing and hybrid inkjet printing technology have attracted much interest for the patterning of PLED devices (see, for example, Science 1998, 279, 1135; Wudl et al, Appl Phys. Lett. 1998, 73, 2561; and J. Bharathan, Y. Yang, Appl. Phys. Lett. 1998, 72, 2660).
In order to contribute to the development of a full-color display, conjugated polymers exhibiting direct color-tuning, good processability and the potential for inexpensive large-scale fabrication have been sought. Poly-2,7-fluorenes have been the subject of much research into blue-light emitting polymers (see, for example, A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, and E. P. Woo, Appl. Phys. Lett. 1998, 73, 629; J. S. Kim, R. H. Friend, and F. Cacialli, Appl. Phys. Lett. 1999, 74, 3084; WO-A-00/55927 and M. Bernius et al. 1, Adv. Mater., 2000, 12, No. 23, 1737).
Active matrix organic light-emitting devices (AMOLEDs) are known in the art wherein electroluminescent pixels and a cathode are deposited onto a glass substrate comprising active matrix circuitry for controlling individual pixels and a transparent anode. Light in these devices is emitted towards the viewer through the anode and the glass substrate (so-called bottom-emission). Devices with transparent cathodes (so-called “top-emitting” devices) have been developed as a solution to this problem. A transparent cathode must possess the following properties: transparency; conductivity; and low workfunction for efficient electron injection into the LUMO of the electroluminescent layer of the device or, if present, an electron transporting layer.
An Example of a top-emitting device is shown in FIG. 6. The top-emitting device comprises a substrate 202 on which an insulating planarization layer 204 is disposed. A via hole is provided in the planarization layer 204 so an anode can be connected to its associated TFT (not shown). An anode 206 is disposed on the planarization layer 204 over which well-defining banks 208 are provided. The anode 206 is preferably reflective. Electroluminescent material 210 is disposed in the wells defined by the banks and a transparent cathode 212 is deposited over the wells and the banks to form a continuous layer.
Inkjet printing of electroluminescent formulations is a cheap and effective method of forming patterned devices. As disclosed in EP-A-0880303, this entails use of photolithography to form wells that define pixels into which the electroluminescent material is deposited by inkjet printing. It is also known to treat the well-defining layer with a fluorine based plasma to decrease the wettability of an upper surface of the well-defining bank layer prior to deposition of the electroluminescent material in a similar manner to that described previous in relation to TFTs.
It is an aim of the present invention to improve on the devices and methods of manufacture described above.